An offensive by the breakaway Al-Qaeda group ISIS in eastern Syria against Islamic rebel factions has killed more than 630 people and uprooted at least 130,000 since the end of April, an anti-regime monitoring group said Tuesday.

The Islamic State of Iraq and Greater Syria’s campaign in the oil-rich Deir al-Zor province appears designed to link the large amount of territory under its control in northern Syria with the ground it also holds across the frontier in neighboring Iraq. The group, which is largely composed of foreign jihadists, has made significant headway over the past six weeks, seizing towns and villages in heavy fighting against the Nusra Front, Al-Qaeda’s official branch in Syria, and an array of Islamist rebel groups.

ISIS has also made major gains in Iraq, where Tuesday its fighters seized parts of Mosul, the country’s second-largest city, scattering security forces and capturing the provincial government headquarters and security bases.

The Britain-based Syrian Observatory for Human Rights said the fighting in Deir al-Zor has killed 634 people since April 30. It said the tally includes 39 civilians, 354 rebel fighters, including many from the Nusra Front, and 241 gunmen from ISIS.

The offensive in Deir al-Zor is part of a broader conflict between ISIS and the jihadists and rebels that has raged across opposition-held northern Syria since early January and killed more than 6,000 people.

This war-within-a-war has been a massive drain on resources and manpower on the opposition in Syria, undermining its fight against President Bashar Assad in the civil war.

Once spread across much of northern Syria, ISIS withdrew many of its far-flung fighters to its stronghold in the northern city of Raqqa earlier this year after other rebel factions, furious with the group’s efforts to impose its hard-line interpretation of Islam, launched an offensive against it.

But ISIS has since consolidated its hold on Raqqa and the surrounding province, although smaller Islamist rebel groups have recently claimed the seizure of a number of villages near the provincial capital and vowed to uproot ISIS from its stronghold.

In the first week of May, fighters from ISIS pushed onward to the neighboring province of Deir al-Zor, blasting their way through towns along the Euphrates River and closing in on the provincial capital, the city of Deir al-Zor.

The Observatory said ISIS now controls most of the northeast bank of the Euphrates, from close to the border with Turkey down to the town of Busayra nearly 320 km to the southeast. The Observatory said ISIS militants Tuesday detained two people from Busayra and took them to an undisclosed location.

ISIS aims to extend its control all the way to the town of Al-Bukamal on the Iraqi border, helping to strengthen links between its Syrian and Iraqi wings, the Observatory’s Rami Abdel-Rahman said.

The group follows Al-Qaeda’s jihadist ideology but its leader Abu Bakr al-Baghdadi has defied orders by Al-Qaeda leader Ayman al-Zawahri to stop fighting in Syria and focus on Iraq.

Rebel groups have periodically announced campaigns against ISIS and Tuesday, rebels in Aleppo province – where ISIS holds a small strip of territory – declared the latest, dubbed the Nahrawan campaign.

They claimed that fierce fighting had claimed the lives of 13 rebels and 35 ISIS fighters, as the rebels seized several small villages in the north of Aleppo province.

A video also emerged, purporting to show ISIS militants in the town of Manbij in rural Aleppo carry out the public execution of two men, although their alleged crime was not disclosed.